The host components required for a compatible interaction between a plant and a pathogen ar uncharacterized. Traditionally, the host has been viewed as a passive partner during this interaction. However, there are probably many host components involved in the precise and measured interaction between a plant and a biotrophic pathogen. The relationship between an obligate biotrophic pathogen and a plant is similar to the beneficial symbiotic relationship between plants and mycorrhizal fungi or nitrogen fixing bacteria. The objective of this study is to identify host factors required for the compatible interaction between Arabidopsis thaliana and the powdery mildew pathogen, Erysiphe cichoracearum. We will use a mutagenic approach to identify mutants with defects in host components required for compatibility. Phenotypically, these mutants will be resistant to the powdery mildew pathogen. The mutants will be characterized and the most interesting gene will be cloned. These studies will provide an insight into a poorly studied are of plant pathology. The information learned about this system may be applicable to other biotrophic pathogens including biotrophic human pathogens like rickettsia and chlamydia. In addition, the study of these mutants may provide insight into plant interactions with beneficial symbionts.